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<!--
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Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
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DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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This code is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License version 2 only, as
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published by the Free Software Foundation. Oracle designates this
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particular file as subject to the "Classpath" exception as provided
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by Oracle in the LICENSE file that accompanied this code.
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This code is distributed in the hope that it will be useful, but WITHOUT
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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version 2 for more details (a copy is included in the LICENSE file that
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accompanied this code).
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You should have received a copy of the GNU General Public License version
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2 along with this work; if not, write to the Free Software Foundation,
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Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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or visit www.oracle.com if you need additional information or have any
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questions.
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-->
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<body>
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<p>
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Nashorn is a runtime environment for programs written in ECMAScript 5.1.
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</p>
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<h1>Usage</h1>
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<p>
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The recommended way to use Nashorn is through the <a href="http://jcp.org/en/jsr/detail?id=223" target="_top">JSR-223
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"Scripting for the Java Platform"</a> APIs found in the {@link javax.script} package. Usually, you'll obtain a
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{@link javax.script.ScriptEngine} instance for Nashorn using:
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<pre>
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import javax.script.*;
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...
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ScriptEngine nashornEngine = new ScriptEngineManager().getEngineByName("nashorn");
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</pre>
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and then use it just as you would any other JSR-223 script engine. See
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<a href="jdk/nashorn/api/scripting/package-summary.html">{@code jdk.nashorn.api.scripting}</a> package
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for details.
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<p>
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<h1>Compatibility</h1>
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Nashorn is 100% compliant with the <a href="http://www.ecma-international.org/publications/standards/Ecma-262.htm"
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target="_top">ECMA-262 Standard, Edition 5.1</a>. It requires a Java Virtual Machine that implements the
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<a href="http://jcp.org/en/jsr/detail?id=292" target="_top">JSR-292 "Supporting Dynamically Typed Languages on the Java
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Platform"</a> specification (often referred to as "invokedynamic"), as well as the already mentioned JSR-223.
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<h1>Interoperability with the Java platform</h1>
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<p>
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In addition to being a 100% ECMAScript 5.1 runtime, Nashorn provides features for interoperability of the ECMAScript
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programs with the Java platform. In general, any Java object put into the script engine's context will be visible from
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the script. In terms of the standard, such Java objects are not considered "native objects", but rather "host objects",
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as defined in section 4.3.8. This distinction allows certain semantical differences in handling them compared to native
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objects. For most purposes, Java objects behave just as native objects do: you can invoke their methods, get and set
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their properties. In most cases, though, you can't add arbitrary properties to them, nor can you remove existing
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properties.
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<p>
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<h2>Java collection handling</h2>
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<p>
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Native Java arrays and {@link java.util.List}s support indexed access to their elements through the property accessors,
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and {@link java.util.Map}s support both property and element access through both dot and square-bracket property
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accessors, with the difference being that dot operator gives precedence to object properties (its fields and properties
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defined as {@code getXxx} and {@code setXxx} methods) while the square bracket operator gives precedence to map
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elements. Native Java arrays expose the {@code length} property.
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<p>
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<h2>ECMAScript primitive types</h2>
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<p>
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ECMAScript primitive types for number, string, and boolean are represented with {@link java.lang.Number},
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{@link java.lang.CharSequence}, and {@link java.lang.Boolean} objects. While the most often used number type is
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{@link java.lang.Double} and the most often used string type is {@link java.lang.String}, don't rely on it as various
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internal optimizations cause other subclasses of {@code Number} and internal implementations of {@code CharSequence} to
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be used.
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<p>
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<h2>Type conversions</h2>
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<p>
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When a method on a Java object is invoked, the arguments are converted to the formal parameter types of the Java method
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using all allowed ECMAScript conversions. This can be surprising, as in general, conversions from string to number will
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succeed according to Standard's section 9.3 "ToNumber" and so on; string to boolean, number to boolean, Object to
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number, Object to string all work. Note that if the Java method's declared parameter type is {@code java.lang.Object},
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Nashorn objects are passed without any conversion whatsoever; specifically if the JavaScript value being passed is of
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primitive string type, you can only rely on it being a {@code java.lang.CharSequence}, and if the value is a number, you
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can only rely on it being a {@code java.lang.Number}. If the Java method declared parameter type is more specific (e.g.
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{@code java.lang.String} or {@code java.lang.Double}), then Nashorn will of course ensure the required type is passed.
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<p>
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<h2>SAM types</h2>
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<p>
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As a special extension when invoking Java methods, ECMAScript function objects can be passed in place of an argument
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whose Java type is so-called "single abstract method" or "SAM" type. While this name usually covers single-method
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interfaces, Nashorn is a bit more versatile, and it recognizes a type as a SAM type if all its abstract methods are
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overloads of the same name, and it is either an interface, or it is an abstract class with
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a no-arg constructor. The type itself must be public, while the constructor and the methods can be either public or
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protected. If there are multiple abstract overloads of the same name, the single function will serve as the shared
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implementation for all of them, <em>and additionally it will also override any non-abstract methods of the same name</em>.
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This is done to be consistent with the fact that ECMAScript does not have the concept of overloaded methods.
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<p>
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<h2>The {@code Java} object</h2>
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Nashorn exposes a non-standard global object named {@code Java} that is the primary API entry point into Java
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platform-specific functionality. You can use it to create instances of Java classes, convert from Java arrays to native
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arrays and back, and so on. The methods on the objects are directly implemented by public static methods on the class
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<a href="jdk/nashorn/internal/objects/NativeJava.html">{@code NativeJava}</a>, see that class for details on what
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functionality is available.
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<h2>Representations of Java types</h2>
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The method <a href="jdk/nashorn/internal/objects/NativeJava.html#type(java.lang.Object,%20java.lang.Object)">
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{@code Java.type(typeName)}</a> takes a name of a type, and returns an object representing a Java type. You can
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use that object to both create new instances of Java classes, as well as to access static fields and methods on them.
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The type object is distinct from the {@code java.lang.Class} object, which represents the reflective run-time type
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identity and doesn't carry i.e. static members. Again, see the link for {@code NativeJava} above for details.
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<h2>Other non-standard built-in objects</h2>
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In addition to {@code Java}, Nashorn also exposes some other non-standard built-in objects:
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<a href="jdk/nashorn/internal/objects/NativeJSAdapter.html">{@code JSAdapter}</a>,
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<a href="jdk/nashorn/internal/objects/NativeJavaImporter.html">{@code JavaImporter},
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<a href="jdk/nashorn/internal/runtime/NativeJavaPackage.html">{@code Packages}.</a>
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</body>
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